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QoS Overview
WRED Slopes
Operation and Configuration
The 7210 SAS provides a single system buffer pool for use by all the queues created in the system.
Each queue provides user an option to configure high-priority WRED slope and a low-priority
WRED slope
The high-priority RED slope manages access to the shared portion of the buffer pool for high-
priority or in-profile packets. The low-priority RED slope manages access to the shared portion of
the buffer pool for low-priority or out-of-profile packets.
By default, the high-priority and low-priority slopes are disabled.
The WRED uses average queue lengths, queue thresholds provisioned, and drop probablility to
calculate the packet's eligibility to be enqueued. The committed portion of the buffer pool is
exclusively used by a queue to enqueue traffic within committed rate.
For the queues within a buffer pool, packets are either queued using committed burst size (CBS)
buffers or shared buffers. The CBS buffers are simply buffer memory that has been allocated to the
queue while the queue depth is at or below its CBS threshold. The amount of CBS assigned to all
queues is dependent upon the number of queues created, the setting of the default CBS as defined
in the policy, and any CBS values set per queue within a QoS policy. However, from a functional
perspective, the buffer pool does not keep track of the total of the CBS assigned to queues serviced
by the pool. CBS subscription on the pool is an administrative function that must be monitored by
the queue provisioner.
For each queue, the amount of access and network buffer pools, the percentage of the buffers that
are to be reserved for CBS buffers is configured by the software (cannot be changed by user). This
setting indirectly assigns the amount of shared buffers on the pool. This is an important function
that controls the ultimate average and total shared buffer utilization value calculation used for
WRED slope operation. The CBS setting can be used to dynamically maintain the buffer space on
which the WRED slopes operate.
When a queue depth exceeds the queue's CBS, packets received on that queue must contend with
other queues exceeding their CBS for shared buffers. To resolve this contention, the buffer pool
uses two WRED slopes to determine buffer availability on a packet by packet basis. A packet that
was either classified as high priority or considered in-profile is handled by the high-priority
WRED slope. This slope should be configured with WRED parameters that prioritize buffer
availability over packets associated with the low-priority WRED slope. Packets that had been
classified as low priority or out-of-profile are handled by this low-priority WRED slope.
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7210 SAS-X and 7210 SAS-R6 OS Quality of Service Guide
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